Lithium-ion secondary batteries are widely adopted as power supplies for small-sized equipment because they have a small volume and a large mass capacity density, and can provide a high voltage. For example, lithium-ion secondary batteries are used as power supplies for mobile equipment such as a mobile phone and a laptop computer. Moreover, in recent years, application of the lithium ion secondary battery not only to small mobile devices but also to large secondary batteries in the field of electric vehicles (EV), electric power storage, or the like where a large capacity with long life is required has been expected, based on concern for environmental problems and improvement in consciousness of energy conservation.
In currently commercially available lithium-ion secondary batteries, one based on LiMO2 (M is at least one of Co, Ni, and Mn) having a layered structure or LiMn2O4 having a spinel structure is used as a positive electrode active material. As a negative electrode active material, carbon materials such as graphite are used. As a voltage of such a battery, a charge-discharge region of 4.2 V or less is mainly used.
On the other hand, it is known that a material in which a part of Mn in LiMn2O4 with Ni or the like is replaced exhibits a high charge-discharge region of 4.5 to 4.8 V versus lithium metal. More specifically, a spinel compound such as LiNi0.5Mn1.5O4 exhibits a high operating voltage of 4.5 V or more because Mn is present in a state of Mn4+ and oxidation-reduction of Ni2+ and Ni4+ is utilized, which is not conventional oxidation-reduction of Mn3+ and Mn4+. Such a material is called a 5 V-class active material, and can achieve an improvement in an energy density by increasing a voltage, and therefore, is expected as a promising positive electrode material.
However, there was a problem in that, when the voltage of the positive electrode becomes high, cycle deterioration of the battery increases due to, for example, the gas generation caused by oxidative decomposition of the electrolyte solution, a by-product generated in associated with the decomposition of the electrolyte solution and elution of metal ions such as Mn and Ni in the positive electrode active material, which deposit on the negative electrode to accelerate deterioration of the negative electrode. In particular, in practical use of a 5 V-class positive electrode, gas generation has been a major obstacle.
As a method for suppressing cycle deterioration and gas generation of a lithium-ion battery, a SEI (Solid Electrolyte Interface) film formation has been conducted on the surface of an active material by adding additives to an electrolyte solution. The SEI film is an electronically insulating body but is considered to have a lithium ion conducting property, and functions to prevent a reaction between the active material and the electrolyte solution. Many of such additives form a film on the negative electrode. However, since a 5 V-class active material is dominantly affected by decomposition of the electrolyte solution in the positive electrode, these additives that form a film on the negative electrode have not obtained a sufficient effect against gas generation.
On the other hand, an attempt for improving battery performances has been conducted by performing surface treatment on a positive electrode active material. For example, there is disclosed a technique of improving battery performances by covering the surface of a positive electrode or a positive electrode active material with a conductive polymer typified by polyaniline by a method of electrolytic oxidation or chemical oxidation (Patent Literature 1 to 3). However, in these Patent Literatures, additives that exhibit an effect of suppressing gas generation of a 5 V-class active material are not specifically described at all.